Method for the preparation of acid-tolerant calcium carbonate fillers and filled paper based on high-lignin-content deinked pulp derived from recycled newspaper

ABSTRACT

The present invention relates to a method for the modification of calcium carbonate fillers to improve their acid-tolerant property and to make their effective use in papermaking of high-lignin-content deinked pulp derived from recycled newspaper. It has been found that modification of calcium carbonate fillers using the combination of a calcium salt with a weak acid can improve the acid-tolerant property of the fillers, and the optical properties of the filled paper, such as brightness, can also be effectively improved.

CROSS REFERENCE TO RELATED PATENT APPLICATIONS

This patent application claims priority from Chinese patent applicationSerial No. 201010531823.9, filed on Nov. 4, 2010.

FIELD OF THE INVENTION

The present invention relates to a method for the preparation ofacid-tolerant fillers and use of the fillers in high-lignin-contentdeinked-pulp-containing papers with the feature that filler modificationwith calcium salt/weak acid can significantly improve the acid-tolerantproperty of the fillers and optical properties of the filled papers.

BACKGROUND OF THE INVENTION

Mineral fillers are widely used in the paper industry to reducemanufacturing cost and to improve the critical properties (such asbrightness and opacity) of paper products. The fillers suitable for usecan include kaolin clay (calcined or hydrous), natural ground calciumcarbonate, precipitated calcium carbonate, talc, titanium oxide, silica,calcium silicate, aluminosilicate, alumina trihydrate, precipitatedcalcium sulphate, sericite, etc, to mention just a few.

Among different types of fillers, calcium carbonate fillers, includingground calcium carbonate (GCC) and precipitated calcium carbonate (PCC),are preferred in many cases due to their low cost and good opticalproperties. As one category of fillers with alkaline characteristic,calcium carbonate can be effectively used in alkaline papermaking toprovide cost savings and optical properties improvement. However, theuse of calcium carbonate fillers in high-lignin-content papers wasdiscouraged by the so called “Fiber Alkaline Darkening” effect. Underacidic or pseudo-neutral papermaking conditions, “Fiber AlkalineDarkening” is very low; however, the dissolution of calcium carbonatefillers under such conditions is significant due to their pooracid-tolerant property.

In order to inhibit the “Fiber Alkaline Darkening” effect, to improvethe acid-tolerant property of calcium carboante fillers, and to enablethe effective use of cost-effective calcium carbonate fillers inhigh-lignin-content paper grades, various methods have been proposed.

Pakarinen, H. and Leino, H., Wochenblatt für Papierfabrikation, 129,953-958 (2003) discloses the use of carbon dioxide to adjust thepapermaking system containing deinked pulp (containing calciumcarbonate) to pseudo neutral, and the benefit of using carbon dioxide inthe production of deinked pulp containing newsprint is discussed.

U.S. Pat. No. 6,540,870 discloses the use of carbon dioxide tosubstantially retard the dissolution of calcium carbonate in apapermaking system.

U.S. Pat. No. 6,540,878 discloses the treatment of calcium carbonatefillers with carbon dioxide for papermaking applications, and thedissolution rate of calcium carbonate is retarded.

U.S. Pat. No. 5,599,388 discloses the treatment of calcium carbonatefillers with aluminum salt (such as aluminum sulfate, aluminum chloride,or polyaluminum chloride), and the modified fillers are acid resistant,which are claimed to be suitable for use in the making of neutral toweakly acid paper.

U.S. Pat. No. 5,043,017 and U.S. Pat. No. 5,156,719 disclose a processfor the preparation of acid-tolerant calcium carbonate fillers by theaddition to finely divided calcium carbonate of one of acalcium-chelating agent and a conjugate base, such as sodiumhexametaphosphate, followed by the addition of a weak acid, such asphosphoric acid. The method for the use of acid-tolerant calciumcarbonate fillers in the making of neutral to acidic paper in order toimprove the optical properties of the paper is also disclosed. Evans, B.and Slozer, M., 5th International Paper & Coating Symposium 43-46 (2003)discloses the process of adding a weak acid (such as phosphoric acid)continuously to the wet end of a paper machine containing calciumcarbonate filler particles and an appropriate chelant/sequestrant (suchas sodium hexametaphosphate), and such a process is claimed to besuitable for enabling the effective use of calcium carbonate fillers ingroundwood papers.

Shen, J. et al., BioResources, 4(3): 1178-1189 (2009) and Shen et al.China Pulp & Paper, 27(10): 13-17 (2008) disclose the use of sodiumsilicate, phosphoric acid, and sodium hexameta-phosphate foracid-stabilization of precipitated calcium carbonate filler, and such afiller modification method can improve the use of precipitated calciumcarbonate in papermaking of deinked pulp derived from recyclednewspaper.

U.S. Pat. No. 5,593,488 discloses an improved form of calcium carbonatewhich is acid resistant to enable its use as a filler material in themaking of neutral to weakly acidic paper, and a process for producingthis acid resistant calcium carbonate is provided. This acid resistantcalcium carbonate comprises a mixture of at least about 0.1 percent,based on the dry weight of the calcium carbonate, of a weak basetogether with at least about 0.1 percent, based on the dry weight of thecalcium carbonate, of a weak acid.

U.S. Pat. No. 6,228,161 and U.S. Pat. No. 7,033,428 disclose a processfor the preparation of acid-stabilized calcium carbonate slurry(suitable for making acid paper) having a pH of less than 7, preferablybetween about 6 and about 7, containing water, calcium carbonate,preferably precipitated calcium carbonate, and an acid-stabilizer of awater soluble calcium salt, a weak acid, a chelating agent, a weak acidcapable of chelating calcium ion, or a mixture thereof. Theacid-stabilizer is present in an amount sufficient to provide an aqueouscalcium carbonate slurry having an increased calcium ion concentrationand an acidic pH. In a typical acid-stabilized calcium carbonate slurryof the invention, the acid-stabilizer is present in an amount sufficientto provide a calcium ion concentration of about 1 millimolar to about 5molar, preferably from about 1 to about 120 millimolar.

Pang, P., Ph.D Thesis of the University of British Columbia (2001),Pang, P. et al., Industrial & Engineering Chemistry Research, 40:2445-2451 (2003), Pang, P. and Englezos, P., Pulp and Paper Canada,104(6): 152-154 (2003), Pang, P. et al., Tappi Journal, 81(4): 188-192(1998), and Pang, P. et al., Appita Journal, 56(2): 122-126 (2003)disclose the use of phosphoric acid in modification of calcium carbonatefillers to improve their acid-tolerant property and make them moresuitable for use in high-lignin-content paper grades.

U.S. Pat. No. 5,913,973 discloses a method for producing a high solidsslurry of an acid-resistant precipitated calcium carbonate for shippingpurposes, and the acid-resistant calcium carbonate is obtained by suchprocesses as modification with sodium aluminate and a weak acid.

U.S. Pat. No. 6,083,317 discloses an improved form of calcium carbonatewhich is acid resistant to enable its use as a filler material in themaking of neutral to weakly acidic paper. This improved form of calciumcarbonate is obtained by modification of calcium carbonate with sodiumsilicate and one or more weak acids or alum.

U.S. Pat. No. 5,000,791 discloses a process for the preparation of anacid-resistant coating and calcium carbonate particles, in which processa slurry of calcium carbonate particles is mixed simultaneously with thesolution of a zinc compound and a solution of a silica-containingsubstance at a temperature of 70°-95° C., the pH being maintained at8-11. This invention also relates to the use of calcium carbonateparticles having an acid-resistant coating in accordance with theabove-mentioned process, as an acid-resistant filler in the productionof paper at pH 4.0-7.0.

U.S. Pat. No. 5,164,006 discloses a process for the modification ofcalcium carbonate fillers with sodium silicate, carbon dioxide, and zincchloride to prepare acid-resistant calcium carbonate fillers suitablefor papermaking applications.

Shen, J. et al., BioResources, 4(4): 1498-1519 discloses the use ofsodium silicate, zinc chloride, sodium hexametaphosphate, and phosphoricacid for the acid-stabilisation modification of precipitated calciumcarbonate filler, and such a process for filler modification can improvethe use of precipitated calcium carbonate filler in papermaking ofdeinked pulp derived from recycled newspaper.

U.S. Pat. No. 4,174,998 discloses a method for filler preflocculationusing a starch phosphate and an organic polymeric retention aid such asa polyacrylamide. The use of a starch phosphate and an organic polymericretention aid for preflocculation of calcium carbonate fillers canenhance their acid-tolerant property, when used in papermaking.

U.S. Pat. No. 3,873,336 discloses a method for modification of calciumcarbonate fillers with a mixture of anionic starch derivative andcationic starch derivative for papermaking application, and such afiller modification process can improve the acid-tolerant property ofcalcium carbonate fillers.

European Patent No. 2,158,359 discloses a filler modification method forpapermaking applications using anionic latex as a modifier, and such amethod can improve the acid-tolerant property of calcium carbonatefillers.

There is still a need to improve the acid-tolerant property of calciumcarbonate fillers and to improve their use in high-lignin-content paperproducts.

SUMMARY OF THE INVENTION

The present invention provides a method for the preparation ofacid-tolerant precipitated calcium carbonate fillers and their use inpapermaking of deinked pulp derived from recycled newspaper.

It has now been found that the use of calcium salt in combination with aweak acid for the modification of calcium carbonate fillers can improvetheir acid-tolerant property significantly, and use of the modifiedfillers in papermaking of high-lignin-content deinked pulp derived fromrecycled newspaper can significantly improve the optical properties ofthe paper products.

An embodiment of the present invention provides a process of improvingacid tolerance of particulate calcium carbonate used as filler inproduction of paper from high-lignin-content deinked pulp derived fromrecycled newspaper, comprising treating the particulate calciumcarbonate by mixing the particulate calcium carbonate with a calciumsalt in the presence of a weak acid.

More particularly, this invention concerns an acid-tolerant calciumcarbonate composition comprising a mixture of calcium carbonate withabout 0.1 to about 30 percent, based on the dry weight of the calciumcarbonate, of a calcium salt, together with about 0.1 to about 10percent, based on the dry weight of the calcium carbonate, of a weakacid. The use of said acid-tolerant calcium carbonate fillers canprovide more significant improvement in paper brightness in comparisonwith the unmodified calcium carbonate.

A further understanding of the functional and advantageous aspects ofthe invention can be realized by reference to the following detaileddescription.

DETAILED DESCRIPTION OF THE INVENTION

Generally speaking, the embodiments described herein are directed tomethods for the preparation of acid-tolerant fillers and use of thefillers in high-lignin-content deinked-pulp-containing papers. Asrequired, embodiments of the present invention are disclosed herein.However, the disclosed embodiments are merely exemplary, and it shouldbe understood that the invention may be embodied in many various andalternative forms. Some features may be exaggerated or minimized to showdetails of particular elements while related elements may have beeneliminated to prevent obscuring novel aspects.

Therefore, specific structural and functional details disclosed hereinare not to be interpreted as limiting but merely as a basis for theclaims and as a representative basis for teaching one skilled in the artto variously employ the present invention. For purposes of teaching andnot limitation, the illustrated embodiments are directed to methods forthe preparation of acid-tolerant fillers and use of the fillers inhigh-lignin-content deinked-pulp-containing papers. As used herein, theterms, “comprises” and “comprising” are to be construed as beinginclusive and open ended, and not exclusive. Specifically, when used inthis specification including claims, the terms, “comprises” and“comprising” and variations thereof mean the specified features, stepsor components are included. These terms are not to be interpreted toexclude the presence of other features, steps or components.

As used herein, the coordinating conjunction “and/or” is meant to be aselection between a logical disjunction and a logical conjunction of theadjacent words, phrases, or clauses. Specifically, the phrase “X and/orY” is meant to be interpreted as “one or both of X and Y” wherein X andY are any word, phrase, or clause.

As used herein the phrase “increasing acid tolerance” means enabling theparticulate fillers to be more resistant to attack by any acids oracidic groups present in an aqueous medium.

As used herein, the phrase “weak acid” means any acids that dissociateincompletely.

The present inventors have discovered that a modification of calciumcarbonate fillers with a calcium salt combined with a modification witha weak acid imparts to the calcium carbonate fillers used forpapermaking (such as precipitated calcium carbonate) improvedacid-tolerant property of the fillers, and the use of the fillers inpapermaking of high-lignin-content deinked pulp can be improvedcorrespondingly. In comparison with unmodified calcium carbonatefillers, the modified calcium carbonate fillers modified according tothe invention disclosed herein have a high degree of acid-resistance,and paper produced containing the modified calcium carbonate fillershave higher optical properties, such as brightness.

While not wishing to be bound by any particular theory as to theoperability of the present invention, it is believed that thecombination of a calcium salt and a weak acid can inactivate the surfaceof calcium carbonate fillers for papermaking applications, and themodifiers can also form a highly buffered system to confer a high degreeof acid-resistance to the calcium carbonate fillers.

The calcium carbonate fillers utilized are preferably finely divided andthey can be either precipitated calcium carbonate or natural groundcalcium carbonate. They can be utilized either as a powder or an aqueousslurry with a solids content of 10% to 60%.

In a first embodiment of the present invention, the calcium salt ispresent in a sufficient amount to provide a highly buffered systemand/or sufficient surface inactivation of the calcium carbonate, and theamount can be in the range from about 0.1 to about 30 percent, based onthe dry weight of the calcium carbonate, preferably from about 0.5 toabout 10 percent, based on the dry weight of the calcium carbonate. Thecalcium salt utilized in the compositions of the present invention canbe any form of calcium salt available as commercial product, such ascalcium sulfate, calcium acetate, calcium nitrate, calcium citrate, acalcium halide, e.g., calcium chloride, and mixtures thereof to mentionjust a few.

In a second embodiment of the present invention, the weak acid ispresent in a sufficient amount to provide a highly buffered systemand/or sufficient surface inactivation of the calcium carbonate, and theamount can be in a range from about 0.1 to about 10 percent, based onthe dry weight of the calcium carbonate, preferably from about 1 toabout 6 percent, based on the dry weight of the calcium carbonate. Theweak acid utilized in the compositions of the present invention can beany form of weak acid available as commercial product, such as, but notlimited to carbonic acid, phosphoric acid, sulfurous acid, or acarboxylic acid, where the carbonic acid is typically provided by theaddition of carbon dioxide.

In a third embodiment of the present invention, the acid-stabilizationmodification with calcium salt and weak acid can be carried out byadding the calcium salt and weak acid either simultaneously orsequentially to the calcium carbonate. The two modifiers (calcium saltand weak acid) can be either added in the form of dry powders or in theform of water solutions. If the two modifiers (calcium salt and weakacid) are added to calcium carbonate sequentially, the addition ofcalcium salt can be followed immediately by the addition of weak acid orthe addition of weak acid can be followed immediately by the addition ofcalcium salt; there can be also be a time delay between the addition ofthe calcium salt and the weak acid or vice versa. The time delay can becarefully adjusted to suit specific applications.

In a fourth embodiment of the present invention, the filler modificationprocess can be carried out at temperatures of 0° C. to 100° C.,preferably 10° C. to 50° C.

In an additional embodiment of the present invention, the fillermodification process can be carried out under different air pressures,and the selection of the suitable pressure is dependent upon thespecific modifiers used. When carbon dioxide is used as the weak acid,the modification process can be carried out under either ambientpressure or elevated pressure, depending the operational viability ofthe process.

In a further embodiment of the present invention, the fillermodification process can be carried out during the manufacturing and/orprocessing process of calcium carbonate fillers. The filler modificationprocess can be carried out on the precipitated calcium carbonate andground calcium carbonate at the plants where the calcium carbonate isproduced. The filler modification can be combined with the manufacturingand/or processing of the calcium carbonate fillers to provide loweredcosts.

The present invention also relates to a method of forming a filled paperwith the papermaking raw materials based on high-lignin-content deinkedpulp derived from recycled newspaper. The papermaking process can becarried out by adding the modified calcium carbonate to the welldispersed pulp followed by draining the furnish on the papermaking wire,pressing, and drying, calendering, etc. The present invention alsorelates to the process for the use of modified calcium carbonate inacidic or pseudo-neutral papermaking, where the “Fiber AlkalineDarkening” effect of the high-lignin-content pulp can be minimized,resulting in the production of paper products with higher brightness incomparison to the use of unmodified calcium carbonate fillers.

The invention will be further illustrated by the following Examples,which are to be considered illustrative of the invention, and notlimited to the embodiments shown.

Example 1

This example demonstrates the effect of the addition of a calcium salton the pH of a 5 percent calcium carbonate slurry at a pressure of oneatmosphere. Calcium chloride solution (with the added amount of 1%,based on the dry weight of the calcium carbonate) was added to theprecipitated calcium carbonate slurry containing 10 g precipitatedcalcium carbonate, to make the total weight of the resultant slurry 200g. After 24 hours aging and 48 hours aging, the pH values of unmodifiedprecipitated calcium carbonate slurry with precipitated calciumcarbonate content of 5 percent were 10.06 and 10.15, respectively. After24 hours and 48 hours aging, the pH values of modified precipitatedcalcium carbonate slurry with precipitated calcium carbonate content of5 percent were 9.01 and 9.57. The addition of calcium chloridesuppressed the dissolution of precipitated calcium carbonate filler,resulting in decreased system pH.

Example 2

This example demonstrates the effect of the addition of a calcium saltand a weak acid on the pH of a 5 percent calcium carbonate slurry at apressure of one atmosphere. Calcium chloride solution and phosphoricacid solution are sequentially added to the precipitated calciumcarbonate slurry containing 10 g precipitated calcium carbonate, to makethe total weight of the resultant slurry 200 g. The pH values of fillerslurries, with precipitated calcium carbonate content of 5 percent,after 24 hours aging and 48 hours aging are shown in Table 1.

TABLE 1 Amount of calcium Amount of pH of the pH of the chloridephosphoric slurry after 24 slurry after 48 Sample (%) acid (%) hoursaging hours aging Unmodified — — 10.06 10.15 precipitated calciumcarbonate Modified 1 — 9.01 9.57 precipitated calcium carbonate Modified1  0.5 7.86 8.08 precipitated calcium carbonate Modified 1 1 7.21 7.52precipitated calcium carbonate Modified 1 2 7.15 7.45 precipitatedcalcium carbonate Modified 1 3 6.86 6.99 precipitated calcium carbonateModified 1 4 6.45 6.69 precipitated calcium carbonate

The results show that the use of calcium chloride/phosphoric acid forthe modification of precipitated calcium carbonate can effectivelydecrease the dissolution of precipitated calcium carbonate in aqueousmedium.

Example 3

This example demonstrates the alum consumption values of modifiedprecipitated calcium carbonate filler prepared using calcium chlorideand phosphoric acid with added amounts of 1% and 4%, respectively, basedon the dry weight of the calcium carbonate. For the measurement of theacid-tolerant property of calcium carbonate fillers, the alumconsumption method mentioned in several publications such as U.S. Pat.No. 5,000,791, U.S. Pat. No. 5,164,006, (which are both incorporatedherein by reference in their entirety) and Jaakkola, P. and Mannu, H.,Nordic Pulp and Paper Research Journal, 16: 113-117 (2001) was used. Forthe alum consumption method, a 4% alum solution was freshly prepared andadded to a 40 mL sample of modified precipitated calcium carbonatefiller slurry at a pace that maintained the pH at 6.50, and theconsumption of alum solution was measured. The alum consumption valuesof modified filler and unmodified filler at different times are shown inTable 2 and Table 3, respectively.

The results demonstrate that filler modification with using calciumchloride and phosphoric acid with added amounts of 1% and 4%,respectively, based on the dry weight of the calcium carbonate, canreduce the alum consumption. From Example 2 and Example 3, it can beconcluded that, filler modification with calcium chloride and phosphoricacid can improve the acid-tolerant property of the calcium carbonate.The results can well support the embodiments of the patent.

TABLE 2 Time (min) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Alum Consumption(mL) 0.3 0.4 0.5 0.5 0.6 0.7 0.8 0.9 1.1 1.4 1.9 2.5 2.8 3.1 3.4

TABLE 3 Time (min) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Alum Consumption(mL) 1.6 2.0 2.5 2.8 3.0 3.1 3.2 3.3 3.6 3.9 4.2 4.5 4.9 5.2 5.5

Example 4

This example demonstrates the effect of acid-tolerant fillers on theoptical properties of the filled paper. When the added amounts ofcalcium chloride and phosphoric acid were 1% and 4%, respectively, theresultant modified precipitated calcium carbonate was used as fillermaterial for handsheet preparation, and high lignin-content deinked pulpderived from recycled newspaper was used. The procedure for handsheetpreparation was reported by Shen, J., BioResources, 4(3): 1178-1189(2009), which is now re-shown as follows:

The mixture of pulp and filler was diluted to 0.5%, and the targetfiller is loading level was controlled to be 15 wt %. Cationicpolyacrylamide solution was added, and the slurry was stirred for 1 min.Diluted alum solution was then added dropwise to adjust the slurry pH to6.50, and the pH-adjusting time was controlled to be 1 min. Handsheetswith target basis weight of 60 g/m² were then prepared using the sheetformer (ZQJ1-B200 mm) produced in China. The handsheets were furtherpressed and flattened for 24 h using the apparatus equipped with thesheet former. The conditions for handsheet preparation were keptconsistent for all of the samples.

The optical properties of the unfilled handsheets, unmodifiedprecipitated calcium carbonate filled handsheets, and modifiedprecipitated calcium carbonate filled handsheets are show in Table 4.

It can be seen from Table 4 that filler modification with calciumchloride and phosphoric acid significantly improved the brightness ofthe filled paper.

TABLE 4 Brightness Sample (% ISO) Unfilled handsheets 61.7 Handsheetsfilled with unmodified filler 67.8 Handsheets filled with modifiedfiller 71.1

As used herein, the terms “about” and “approximately”, when used inconjunction with ranges of dimensions of particles, compositions ofmixtures or other physical properties or characteristics, is meant tocover slight variations that may exist in the upper and lower limits ofthe ranges of dimensions so as to not exclude embodiments where onaverage most of the dimensions are satisfied but where statisticallydimensions may exist outside this region. It is not the intention toexclude embodiments such as these from the present invention.

The foregoing description of the preferred embodiments of the inventionhas been presented to illustrate the principles of the invention and notto limit the invention to the particular embodiment illustrated. It isintended that the scope of the invention be defined by all of theembodiment encompassed within the following claims and theirequivalents.

REFERENCES

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1. A process of improving acid tolerance of particulate calciumcarbonate used as filler in production of paper from high-lignin-contentdeinked pulp derived from recycled newspaper, comprising treating theparticulate calcium carbonate by mixing the particulate calciumcarbonate with a calcium salt in the presence of a weak acid.
 2. Theprocess according to claim 1 wherein the calcium salt is selected fromthe group consisting of calcium sulfate, calcium acetate, calciumnitrate, calcium citrate, a calcium halide, and mixtures thereof.
 3. Theprocess according to claim 2 wherein the calcium halide is calciumchloride.
 4. The process according to claim 1 wherein the weak acid isselected from the group consisting of carbonic acid, phosphoric acid,sulfurous acid, citric acid, acetic acid, oxalic acid, a carboxylicacid, and mixtures thereof, and wherein the carbonic acid is provided byaddition of carbon dioxide to the particulate calcium carbonate.
 5. Theprocess according to claim 1 wherein the calcium salt is present in anamount in a range from about 0.1 to about 30 percent, based on the dryweight of the calcium carbonate.
 6. The process according to claim 1wherein the calcium salt is present in an amount in a range from about0.5 to about 10 percent, based on the dry weight of the calciumcarbonate.
 7. The process according to claim 1 wherein the weak acid ispresent in an amount in a range from about 0.1 to about 10 percent,based on the dry weight of the calcium carbonate.
 8. The processaccording to claim 1 wherein the calcium salt and the weak acid areadded to the particulate calcium carbonate either simultaneously orsequentially to the calcium carbonate.
 9. The process according to claim1 wherein the calcium salt is selected from the group consisting ofcalcium sulfate, calcium acetate, calcium nitrate, calcium citrate, acalcium halide, and mixtures thereof.
 10. The process according to claim9 wherein the calcium halide is calcium chloride, and mixtures thereof.11. The process according to claim 9 wherein the calcium salt and theweak acid are added to the particulate calcium carbonate in the form ofdry powders or in the form of aqueous solutions.
 12. The processaccording to claim 1 wherein the calcium salt and the weak acid areadded to the particulate calcium carbonate sequentially, and whereinweak acid is added first followed by the addition of the calcium salt.13. The process according to claim 1 wherein the calcium salt and theweak acid are added to the particulate calcium carbonate sequentially,and wherein calcium salt is added first followed by the addition of theweak acid.
 14. The process according to claim 1 wherein the calcium saltand the weak acid are added to the particulate calcium carbonate at thesame time.
 15. The process according to claim 1 wherein the calcium saltand the weak acid are mixed with the particulate calcium carbonate in asolution at a temperature in a range from about 0° C. to about 100° C.16. The process according to claim 1 wherein the calcium salt and theweak acid are mixed with the particulate calcium carbonate in a solutionat a temperature in a range from about 10° C. to about 50° C.
 17. Apaper product, produced from high-lignin-content deinked pulp derivedfrom recycled newspaper, incorporating particulate calcium carbonatefiller which has been modified according to the method of claim 1.